The present invention relates to valve metal powders and other metal powders of acceptable quality for use in such applications as capacitors, sputter targets, and the like. More particularly, the present invention relates to methods of surface passivation metal powders.
Valve metals, e.g., tantalum and niobium, are generally extracted from their ores in the form of powders. Tantalum powders, for example, that are suitable for use in high performance capacitors, can be produced by chemical reduction, such as sodium reduction, of potassium fluorotantalate. In this process, the potassium fluorotantalate is recovered from processed ore in the form of a dry crystalline powder. The potassium fluorotantalate is melted and reduced to tantalum metal powder by sodium reduction. The tantalum powder formed is then water washed and acid leached, as described, for example, in U.S. Pat. Nos. 6,312,642, and 5,993,513, which are incorporated in their entireties herein by reference. The tantalum is then dried, resulting in what is known as a basic lot powder.
Typically, the basic lot powder is subjected to a heat treatment or thermal agglomeration step and then passivated and stabilized to obtain a powder cake that is subsequently ground up into a powder. A deoxidation step using an oxygen getter is then performed. After the deoxidation step, the tantalum powder can again be passivated to form a passive oxide coating on its surface to form stabilized powder particles. Other techniques used in the processing of tantalum powder to improve the performance characteristics of the finished products made from the metal powder include reacting small quantities of modifying agents to the tantalum powder. A range of additives or “dopants” have been used, including nitrogen, silicon, phosphorous, boron, carbon, and sulfur. Nitriding, for example, can occur between or during any of the aforementioned processing steps. The processed tantalum powder can then be pressed into a pellet and sintered for subsequent processing by capacitor anode manufacturers for example. Another example of a use of the tantalum powder is the isostatic consolidation of the tantalum powder to produce metal articles such as sputter targets.
As mentioned, performance characteristics of the products made from the metal powder can be related to microstructure characteristics of the metal powder. Of particular interest, capacitance and DC leakage of metallic capacitors can be related to the specific surface area of the metal powder used to form the sintered metal body. Greater net surface area can be achieved, of course, by increasing the quantity (grams) of metal per pellet; but, cost and size considerations have dictated that development be focused on means to increase the specific area of the metal powder, that is, to increase volumetric efficiency. Due to the very fine particle size and high surface area, electrolytic capacitor grade metal powders such as tantalum and niobium need to be passivated to prevent a violent reaction upon exposure to atmospheric oxygen that can possibly result in combustion.
Conventional techniques to passivate tantalum and niobium particles involve controlled exposure to atmospheric air in a gradual or a step-wise increase in pressure. Depending on the surface area of the powder, multiple vent/evacuation cycles may be needed at each pressure step to equilibrate the surface with oxygen partial pressure. A primary reason for vent/evacuation cycles is that atmospheric air consists of approximately only 20 wt. % oxygen, with the balance being predominantly nitrogen. Residual nitrogen that is trapped between the powder particles is preferably evacuated before further venting of fresh air. Because the capacitor powder industry is moving toward ever higher surface area of the particles, the necessary number of vent/evacuation cycles has also increased, adding costs and production time for producing suitable metal powders.
Accordingly, a need exists to reduce the number of vent/evacuation cycles needed to surface passivate capacitor grade metal powders, and/or to eliminate the need altogether for the evacuation step in passivating metal powders.